X-ray lithograph (U.S. Pat. No. 3,743,842, July 3, 1973) is a technique in which a pattern in an x-ray attenuator located on a membrane that is relatively transparent to x rays, is replicated in a radiation sensitive film (usually called a resist) by passing x rays through, such that the shadow of the attenuator pattern is cast into the resist. The combination of membrane and patterned attenuator is called the x-ray mask. Usually, the exposure of the resist by the x-rays results in a change in solubility such that after development in a solvent the pattern of the x-ray attenuator is replicated in the resist as a relief image.
In x-ray lithography, diffraction is significantly reduced relative to ultraviolet lithography because of the short wavelength of the x rays (in the range 0.4 to 5 nm). However, diffraction cannot be ignored altogether, especially as the need for deep submicron linewidths, and sub 0.1 .mu.m linewidths, arises. Diffraction, as well as penumbral edge blurring due to the finite source size, can be reduced by reducing the mask-to-sample gap, S. In fact, in our research laboratory practice of x-ray lithography this gap is commonly made zero. However, in industrial practice, one would like to have a relatively large gap, for example in the range 3 to 50 .mu.m. A non-zero gap is desirable so as to avoid mutual mask/sample damage such as scratching. The purpose of this invention is to provide a novel configuration of x-ray mask in which the deleterious effects of diffraction are significantly reduced relative to conventional x-ray masks. This invention will allow one either to use a larger gap while achieving a given minimum linewidth, or, for a given gap, to achieve a smaller minimum linewidth. This invention also provides improved control of linewidth and improved latitude of processing.
The x-ray shadow cast onto a surface (so called recording surface) by an x-ray attenuator pattern is a faithful replica if the gap, S, between the attenuator and the recording surface is close to zero. At the edge of any feature in the pattern, the slope of the intensity profile is extremely steep. If, however, the gap, S, is increased significantly, the intensity profile will be changed as a result of diffraction. Specifically, the slope of the intensity profile will be less steep. The invention described here provides a means for steepening the slope of the intensity profile at the edges of features in x-ray lithographic replication relative to the slope obtained with a conventional x-ray mask. A steeper slope is a highly significant advantage because it permits improved linewidth control.
This invention results from the recognition that the x ray wave that passes through the x-ray attenuator experiences a shift in phase relative to the x ray wave that passes through the open, non-attenuating, portions of an attenuator pattern, and that if this relative phase shift is adjusted to have a value close to .pi. radians, some of the deleterious effects of diffraction will be suppressed. In particular, the slope of the intensity profile will be steeper than if the relative phase difference were other than .pi. radians.